Freshwater lakes are rapidly losing oxygen due to global warming — even faster than the world’s oceans, a new study warns.
Researchers at Rensselaer Polytechnic Institute in New York examined oxygen levels of freshwater lakes in the temperate zone, which spans 23 to 66 degrees latitude north and south.
The oxygen content of the lakes in this zone has dropped 5.5 percent at the surface and 18.6 percent in deep waters since 1980, they found.
Freshwater lakes are bodies of standing, unsalted water surrounded by land and are vital water sources for humans and microorganisms.
A decrease in freshwater oxygen therefore threatens biodiversity and the quality of drinking water for humans.
Oxygen levels in the world’s temperate freshwater lakes are declining faster than in the oceans, say researchers at Rensselaer Polytechnic Institute in New York. Pictured, Lake Michigan
WHAT ARE FRESH WATER LAKES?
Freshwater lakes are bodies of standing, unsalted water surrounded by land.
Freshwater lakes provide benefits to people such as water supply, fishing, flood mitigation, and recreational purposes.
Loch Lomond, a short train ride from Glasgow, is an example of a freshwater loch in the United Kingdom.
However, freshwater is a finite resource — only 3 percent of all the water on Earth is freshwater, according to the WWF.
Freshwater is threatened by overdevelopment, polluted runoff and global warming.
While lakes make up only about three percent of the Earth’s land area, they also contain a disproportionate concentration of the planet’s biodiversity.
“The implications of decreasing oxygen in freshwater are huge,” said study author Kevin Rose, a professor at Rensselaer Polytechnic Institute.
“All complex life depends on oxygen – it’s the support system for aquatic food webs. And if you start to lose oxygen, you have the potential to lose species.
“Lakes lose oxygen 2.75 to 9.3 times faster than the oceans — a decrease that will affect the entire ecosystem.”
When oxygen levels drop, bacteria that thrive in oxygen-deprived environments, such as those that produce the potent greenhouse gas methane, begin to multiply.
This may mean that lakes release greater amounts of methane into the atmosphere as a result of oxygen loss — a devastating double whammy.
“Ongoing research has shown that oxygen levels in the world’s oceans are falling rapidly,” said Curt Breneman, dean of the School of Science at Rensselaer Polytechnic Institute.
“This study now proves that the problem is even greater in freshwater, threatening our drinking water supplies and the delicate balance that allows complex freshwater ecosystems to thrive.
The temperate zone occupies 23 to 66 degrees latitude north and south. It is between the polar and tropical zones
“We hope this finding makes efforts to address the increasingly adverse effects of climate change more urgent.”
For their study, the researchers analyzed more than 45,000 dissolved oxygen and temperature profiles collected since 1941 from nearly 400 lakes around the world.
Most, but not all, long-term records were collected in the temperate zone, which lies between the tropical and polar zones.
Lake Nydia in Ontario, Canada. Study author Kevin Rose said ‘the implications of decreasing oxygen in freshwater are enormous’
As surface water temperatures increased by 0.68°F (0.38°C) per decade, dissolved oxygen concentrations in surface waters decreased by 0.11 milligrams per liter per decade.
“Oxygen saturation, or the amount of oxygen that water can hold, decreases as the temperature rises,” said Professor Rose.
“That’s a well-known physical relationship and it explains most of the trend in surface oxygen that we’re seeing.”
In addition to biodiversity, the concentration of dissolved oxygen in aquatic ecosystems affects greenhouse gas emissions, nutrient bio-geochemistry and human health. Freshwater is threatened by many forces, including overdevelopment, polluted runoff and global warming
However, contrary to this intuition, some lakes experienced simultaneously increasing concentrations of dissolved oxygen and warming temperatures.
These lakes were typically more polluted with nutrient-rich runoff from agricultural and developed watersheds and have high chlorophyll concentrations.
These lakes also tended to cross a threshold where cyanobacteria, a type of microscopic organism, begins to dominate.
Cyanobacteria, which boost oxygen in the water, can create toxins when they bloom, in the form of harmful algal blooms, or HABs.
“The fact that we see an increase in dissolved oxygen in those kinds of lakes may be an indicator of widespread increase in algal blooms, some of which produce toxins and are harmful,” Rose said.
Overall, the changes are worrisome for their potential impact on freshwater ecosystems and for what they suggest about environmental change in general, according to lead author Stephen F. Jane.
“Lakes are indicators or ‘sentinels’ of environmental change and potential environmental threats because they respond to signals from the surrounding landscape and atmosphere,” he said.
“We found that these disproportionately more biodiverse systems are changing rapidly, indicating to what extent the ongoing atmospheric changes are already impacting ecosystems.”
The study was published today in Nature.
OXYGEN LOSS IN FRESH WATER LAKES: DEEP WATERS VS. SURFACE WATER
Since 1980, the oxygen content of lakes in the temperate zone has decreased by 5.5 percent at the surface and 18.6 percent in deep waters, researchers at Rensselaer Polytechnic Institute revealed.
In other words, the oxygen loss is more pronounced in deep waters (where the water temperature has remained largely stable) than in surface waters.
This is probably due to rising surface water temperatures and a longer warm period per year.
Warming surface water in combination with stable deep water temperatures increases the difference in density between these layers, also known as ‘stratification’.
The stronger this layering, the smaller the chance of mixing between layers.
As a result, oxygen in deep waters is less likely to be replenished during the warm stratified season because oxygenation usually comes from processes that occur near the water’s surface.
“The increase in stratification makes mixing or renewing oxygen from the atmosphere to deep waters more difficult and less frequent, with the result that the dissolved oxygen in deep water drops,” says Rose.